SUBSTATION SECURITY AND PROTECTION DURING EARTHQUAKE BASIC INFORMATION AND TUTORIALS

RELATIONSHIP BETWEEN EARTHQUAKE AND
SUBSTATION

To secure and protect substation
equipment from damage due to a seismic event, the relationship
between earthquakes and substation components must first be
understood. Earthquakes occur when there is a sudden rupture along a
preexisting geologic fault.

Shock waves that radiate from the
fracture zone amplify, and depending on the geology, these waves will
arrive at the surface as a complex set of multifrequency vibratory
ground motions with horizontal and vertical components.

The response of structures and
buildings to this ground motion depends on their construction,
ductility, dynamic properties, and design. Lightly damped structures
that have one or more natural modes of oscillation within the
frequency band of the ground motion excitation can experience
considerable movement, which can generate forces and deflections that
the structures were not designed to accommodate.

Mechanisms that absorb energy in a
structure in response to its motion can help in damping these forces.
If two or more structures or pieces of equipment are linked, they
will interact with one another, thus producing a modified response.

If they are either not linked, or
linked in such a way that the two pieces can move independently —
an ideal situation — then no forces are transferred between the two
components. However, recent research has shown that even a
well-designed link may contribute to the response of the equipment or
structure during a seismic event.

For electrical reasons, most pieces of
substation power equipment are interconnected and contain porcelain.
Porcelain is a relatively brittle, low-strength, and low-damping
material compared with steel. Furthermore, unless instructed to do
otherwise, construction personnel will install conductors with little
or no slack, which gives the installation a neat and clean look.

This practice does not allow for any
freedom of movement between components. When the conductor is
installed with little or no slack, even small differential motions of
one piece of equipment can easily impact an adjacent piece of
equipment.

This is because each piece of
interconnected equipment has its own frequency response to an
earthquake. While the equipment at one end of a tight conductor line
is vibrating at 1 Hz, for example, the other piece of equipment at
the other end of the conductor is “trying” to vibrate at, say, 10
Hz.

It is easy to see that when they
vibrate toward each other, the line will go slack. When they vibrate
away from each other, the line will suddenly snap tight, which will
impact the equipment. This is a well-documented occurrence.

Usually, the larger, more massive
equipment will pull the smaller, weaker equipment over. Substation
equipment with natural frequencies within the range of earthquake
ground motions are especially vulnerable to this type of damage by
seismic events.